There are a number of surgical techniques for treating a clouding of the lens, which is referred to as a cataract in medicine. The most common technique is phacoemulsification. Here, a hollow needle vibrating in the ultrasonic range and including a cutting edge at its front end is used to shatter (emulsify) the clouded eye lens into such small pieces that these pieces can be suctioned away through the hollow needle. Subsequently, the surgeon inserts an artificial lens as a replacement for the lens shattered in this way. An essential component when carrying out such phacoemulsification is a handpiece including the aforementioned hollow needle. The required ultrasonic vibrations for shattering the clouded eye lens can be produced in such a way that the handpiece is provided with piezoceramic elements. If a voltage is applied to these piezoceramic elements, a change in length can be caused due to the piezoelectric effect, and so a needle connected to the piezoceramic elements can be deflected in the longitudinal direction.
In order to achieve complete emulsification of the eye lens and hence a high effectiveness in the shortest possible time, it is useful to move the hollow needle with the largest possible amplitudes. This can be performed in such a way that the piezoelectric elements are operated in the region of the resonant frequency of the handpiece. In the unloaded state, the resonant frequency of a handpiece with a hollow needle can be determined very precisely. However, as soon as the needle comes into contact with the lens to be emulsified, the mass conditions change and so there is a shift in the resonant frequency. In order to respectively operate such a handpiece in the region of the resonant frequency where possible, U.S. Pat. No. 6,997,935 B2 proposes to detect the phase between the applied voltage and the flowing current for operating the piezoelectric elements and to regulate it in such a way that, in accordance with the equation P=U·I·cos φ, a power which is as close to the maximum as possible is achieved. So that the factor cos φ assumes a value that is as large as possible, cos φ=1 or φ=0 must apply. Such a situation is given in the case of resonance. However, if there is a shift in the resonant frequency, for example due to change in the mechanical load on the cutting tip, the phase angle φ no longer equals 0, but rather lies in the range between 0 and −π/2 or 0 and +π/2. In accordance with U.S. Pat. No. 6,997,935 B2, after detecting the phase angle φ, the excitation frequency is regulated in such a way that the excitation frequency of the cutting tip corresponds to the natural frequency ω0.
A cause for the shift in the resonant frequency lies not only in a modified load due to the lens fragments (change in mass) but also in the heating of the handpiece during relatively long operation and in aging of the piezoceramic elements and hence changes in the physical properties thereof. These parameters can superpose arbitrarily. A disadvantage of this is that there has to be constant updating so that the piezoceramic elements vibrate precisely at their resonant frequency. A further disadvantage lies in the fact that a plurality of successive measurement points of the voltage and current profile always have to be detected over time in order to determine the phase angle. This causes relatively slow regulation. Consequently, the handpiece does not really vibrate at its resonant frequency despite such large metrological and regulatory outlay. Rather, there always is a significant time delay in the adaptation to the respective current resonant frequency due to the slow regulation.
If the piezoceramic elements of the handpiece are actuated for a relatively long time during a phacoemulsification, there is heating not only of the piezoceramic elements, but, in the case of a sufficiently long operation, the surroundings of the hollow needle actuated by the piezoceramics are also heated to such an extent that the cornea, which is pierced for phacoemulsification, can burn in the vicinity of the needle. Since such injury must be avoided at all costs, it is conventional for the operation of the piezoceramic elements to be interrupted for a predetermined period of time. In the operating breaks created thus, no ultrasound energy is supplied to the piezoceramic elements, and so the hollow needle and its surroundings can cool down. A disadvantage here is that there needs to be relatively complicated actuation of the piezoceramic elements so that these can be in a vibrating state or in a pause state during predetermined periods of time.